Light Hydrocarbon Gases in Guaymas Basin Hydrothermal Fluids: Thermogenic Versus Abiogenic Origin

Light hydrocarbon gases (methane through pentane) in high-temperature hydrothermal fluids of the Guaymas basin, Gulf of California, are predominantly derived from thermocatalysis of organic carbon in sediments intruded by mid-ocean ridge volcanic rocks. The spectrum of C1-C5 gases in Guaymas basin hydrothermal fluids is characterized by a preponderance of alkanes, with essentially no alkenes present. Comparison with high-temperature fluids from 21°N on the East Pacific Rise, where geochemical evidence indicates a distinct lack of thermogenic gas and where methane is derived abiogenically from the basalts, shows that the 21°N hydrocarbon gases are characterized by a prominent ethylene signature. Stable isotope compositions of Guaymas basin methane differ from 21 °N gas; Guaymas basin has δ13C values of –43 to –51 ‰ vs. PDB, compared to 21°N where values as high as – 15 ‰ have been measured. These differences reflect the different origins of Ught hydrocarbon gases in the two systems, with Guaymas methane being overwhelmingly of an organic, thermogenic derivation and showing no evidence of the abiogenic, basalt-derived gas that characterizes the 21°N environment. Carbon isotope analyses of dissolved inorganic carbon in Guaymas basin hydrothermal fluids indicate that high-temperature hydrocarbon oxidation may be an important process acting in conjunction with high-temperature thermocatalytic hydrocarbon generation.